This invention pertains generally to polymer electrolyte membrane fuel cells and more particularly to an apparatus and method for disposal of water.
In the normal operation of a polymer electrolyte membrane xe2x80x9cPEMxe2x80x9d fuel cell, liquid water forms at the cathode side of the fuel cell. There are two sources for this liquid water. One source is the oxidizing gas being fed to the cathode side of the fuel cell. This gas is generally moisturized before being supplied to the cathode and some of this moisture condenses. The other source is an electrochemical reaction occurring on the cathode side which produces water. Discharging oxidizing gas has a limited capacity to carry away to outside the cell the liquid water formed at the cathode side.
When the quantity of liquid water formed on the cathode side exceeds the liquid water carried away by the oxidizing gas, there is a condition called a xe2x80x9cflooded statexe2x80x9d or xe2x80x9cflooding.xe2x80x9d In a flooded state, the liquid water remains on the surface of the cathode electrode and obstructs the dispersion of the oxidizing gas onto the surface of the cathode. This in turn results in a drop in the cell""s voltage and amperage output. Ultimately, flooding will stop the cell""s operation.
Similarly, the water accumulation can also take place in the anode plates of fuel cells. The anode flow is normally humidified before it is introduced to the anode plates. During operation when anode gas is consumed by chemical reaction of the fuel cell, the water content of the anode flow condenses to liquid water. To avoid flow blockage the condensed water has to be removed from the anode plates.
One conventional water removal technique is wicking, or directing the accumulated water away from the cathode using capillaries incorporated in the cathode. Another related water removal technique employs screens or meshes within the cathode to conduct water away from the catalyst layer. Still another go conventional water removal technique is to incorporate hydrophobic substances, such as polytetrafluoroethylene (trade name Teflon RTM,) into the cathode sheet material to urge accumulated water away from the cathode. This type of apparatus has the disadvantages of being limited in quantity of liquid water that can be removed from the flowfield and being limited in mass transfer of cathode gas to the polyelectrolyte membrane of the fuel cell.
It is known in the art to remove water at the cathode side by utilizing an electrode layer comprised of a porous base area with water repellency and plurality of penetration areas higher in water permeability scattered over or formed through the base area. This facilitates the oozing of water generated on the catalytic layer of the PEM fuel cell into the gas channels through the areas of higher permeability. These types of apparatus have the disadvantage of focusing on removing water at the catalytic area adjacent to the polyelectrolyte membrane; being limited in the quantity of water that can be removed and being limited in mass transfer of reactant gas to the polyelectrolyte membrane.
It is known in the art that an interdigitated flowfield will provide means for the reactant gases to flow through the gas diffusion layer (GDL). In this flowfield, the flow channels have dead-ends. The pressure difference between the inlet flow and exit flow in a plate provides the pressure head to force reactant gases to flow through the GDL. This flowfield configuration allows flow of reactants through a larger area of the GDL and provides a convection transport of reactants to support the reaction, improve the efficiency, and eliminate or reduce the mass transport limited operation. Also, flow of reactants through the GDL helps sweep the water produced in the GDL, and consequently, enhance the fuel cell operation.
It is also known in the art to remove fluid at electrodes by positioning a porous support layer near and in fluid communication with each electrode to facilitate fluid transport to and away from each electrode. The porous support layer includes hydrophobic pores and hydrophilic pores integrated throughout the layer. The fuel and oxidizing gasses are supplied through the hydrophobic pores and water is removed through the hydrophilic pores. This apparatus has the disadvantages of focusing on removing water at the electrode area; being limited in quantity of water that can be removed and being limited in mass transfer of cathode gas to the polyelectrolyte membrane.
A conventional fuel cell is comprised of a stack of PEM fuel cells. Such a conventional fuel cell may require a cooling plate for every cell. Reference is made to FIG. 2 in U.S. Pat. No. 5,840,414 and FIG. 3 in U.S. Pat. No. 5,853,909. To support the capillary action in the porous layers these patents require a cooling plate for every cell. This plurality of cooling plates increases the weight and volume of the fuel cell.
Accordingly, there exists a need for an apparatus and method to efficiently remove water produced at the cathode plates of a PEM fuel cell which enhances the flow of cathode gas to the catalytic area and avoids the loss of cathode gas. Further, there is a need for a fuel cell of reduced weight and volume. The present invention satisfies these needs, as well as others, and generally overcomes the presently known deficiencies in the art.
The present invention is directed to apparatus and method for disposal of water in an electrochemical fuel cell. The present invention of water removal technique can equally be used for anode or cathode plates. Taught here is an interdigitated flowfield with a gas block mechanism for water removal in fuel cells.
One aspect of the present invention is a cathode plate assembly for use with a cathode gas in polyelectrolyte membrane fuel cell. The assembly is comprised of the following major components: There is a cathode plate having a first major surface and a second oppositely opposed major surface (see FIG. 1A). Within the first major surface is a flow field comprised of feed side interdigitated channels and exhaust side interdigitated channels that are in an interdigitated configuration. During the operation of the fuel cell there is flow of cathode gas from feed side interdigitated channels to exhaust side interdigitated channels.
Positioned adjacent to the flow field, are one or more porous gas block mediums. These porous gas blocks have pores sized such that water is sipped off to the outside of the flow field by capillary flow and cathode gas is blocked from flowing through the medium. There is a gas diffusion layer in close contact and over the first surface of cathode plate with its flow field. On the second major surface of the cathode plate can be an anode flowfield or a coolant flowfield. In both cases, the gas block is either in fluid communication with the coolant flowfield or a coolant manifold. Alternatively, the second major surface of the cathode plate may include an anode flowfield for delivering hydrogen, or a hydrogen mixture, to the fuel cell. In this case, the gas blocks are generally in communication to a liquid water manifold generally positioned at the perimeter of the plate for the purpose of delivering water to those plates that do have liquid water channels or to the second surface of the cathode plates. Note that the water channels can further serve the purpose of cooling channels, but are not required to have this function.
Similarly, an anode plate of design analogous to the cathode plate may encounter water accumulation. A gas block mechanism can also be used in an anode plate to help sweep water-to-water channels similar to those described above.
Another aspect of the present invention is a cathode plate assembly for use in a fuel cell with a pressurized cathode gas, and a pressurized coolant or an anode flowfield, comprised of the following components: A cathode plate having a first major surface and a second oppositely opposed major surface. A flow field for pressurized cathode gas within the first major surface having a feed side having a feed side internal plenum in fluid communication with one or more feed side interdigitated channels having width and with dead-ends and an exhaust side having an exhaust side internal plenum in fluid communication with a plurality of exhaust side interdigitated channels having width and with dead-ends, where the feed side and exhaust side interdigitated channels are in an interdigitated configuration defining land between the interdigiated channels such that in the operation of the fuel cell there is flow of cathode gas by convection from feed side interdigitated channels to exhaust side interdigitated channels.
A multiplicity of porous gas block mediums positioned in the cathode plate adjacent to each feed side interdigitated channel at points where liquid water forms during the operation of the fuel cell and having pores sized such that liquid water in the feed side interdigitated channels is sipped off by capillary flow and cathode gas is blocked. A gas diffusion layer closely positioned over the first surface of the cathode plate and flow field therein. A water channel at the second major surface of the cathode plate, or a water manifold at the perimeter of the cathode plate, in fluid communication with each porous gas block medium through which pressurized water flows and where the pressure of the water and gas in feed side interdigitated channels is greater than the pressure of coolant in the water channel such that liquid water flows from each porous gas block medium into the cooling channel or a manifold whereby the water may be the coolant.
Another aspect of the present invention is a cathode plate assembly for use in a fuel cell with a pressurized cathode gas and a pressurized water comprised of the following components: A cathode plate that is a regular four sided polygon having a first major surface, a second oppositely opposed major surface and a first and third and second and fourth oppositely opposed pairs of edges where the first edge is at a higher gravitational potential energy than the third edge. A flow field for pressurized cathode gas (with a few atmospheres pressure) within the first major surface having a feed side internal plenum running parallel to the first edge of the cathode plate; a plurality of feed side interdigitated channels having widths that are in fluid communication with and substantially perpendicular to the feed side internal plenum that extend downward toward the third edge and terminate at dead-ends; an exhaust side internal plenum running parallel to the third edge of the cathode plate; a plurality of exhaust side interdigitated channels having widths that are in fluid communication with and substantially perpendicular to the exhaust side internal plenum that extend upward toward the first edge and terminate at dead-ends and which are interdigitated between the feed side interdigitated channels so as to define land between the interdigitated channelswhere the ratio of pressure drop per unit length of the cathode gas flow over the land between a feed side interdigitated channel and a neighboring exhaust side interdigitated channel and the feed side interdigitated channel cathode gas flow is in the range of about 8:1 to about 15:1. As a specific example, for a GDL, Toray TGPH-090 with a porosity of about 85%, the land width between the interdigitated channels, with 8 inch length, is in the range of about 2 to about 3 times larger than the width of the interdigitated channels such that cathode gas flows by convection from the feed side interdigitated channel to a neighboring exhaust side interdigitated channel.
A multiplicity of porous gas block mediums positioned adjacent to the dead-ends of each of the feed side interdigitated channels having a bubble point in the range of between about 10 psig to about 70 psig such that liquid water is sipped out of the feed side interdigitated channels by capillary flow and cathode gas is blocked. A gas diffusion layer closely positioned over the first surface of the cathode plate and flow field therein. A water channel at the second major surface of the cathode plate, or a manifold at the cathode plate perimeter, in fluid communication with each porous gas block medium through which pressurized coolant flows and where the pressure of the cathode gas in feed side interdigitated channels is greater than the pressure of water in water channels (but does not exceed the bubble point pressure) such that liquid water flows from the porous gas block medium to the water channel or manifold.
A further aspect of the present invention is a method of removing water accumulating at the cathode sides of a stack of electrochemical fuel cells with coolant plates having a coolant flow. The method is comprised of the following steps: installing at the cathode side of each of the fuel cells in the stack any one of the cathode plate assemblies of the present invention; manifolding the single water channel of each of the cathode plate assemblies to the coolant flow that feeds coolant plates; and generating electricity by passing anode and cathode gas through the fuel cell such that the produced liquid water at the cathode sides of each fuel cell is sipped off to the outside of the plate through porous gas mediums of the cathode plate assemblies.
The previously described versions of the present invention have many advantages which include enhanced removal of produced water with the avoidance of the loss of cathode gas and enhanced cathode gas diffusion to the membrane. A fuel cell comprised of PEM cells employing this apparatus is light weight and small in volume.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following description, appended claims and accompanying drawings.